Allow setting the `bulk` index size via SearchPress GUI

The SearchPress index operation, when done via GUI, ignores any custom bulk size added via a plugin. The suggestion is to allow setting a bulk size in the plugin GUI.

# Acceptance Criteria

- Add a bulk size field to the GUI that will be used when performing a full sync from the admin instead of the default value of 500.
- Add a label to this field that describes the default value (500) and recommendations for minimum and maximum values (e.g., if you are having problems with indexing, try a lower number, and we don't recommend setting this higher than 2000).